Re: [syzbot] KASAN: use-after-free Write in nft_ct_tmpl_put_pcpu

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On 8/9/21 11:39 PM, Florian Westphal wrote:
Pavel Skripkin <paskripkin@xxxxxxxxx> wrote:
I think, there a missing lock in this function:

	for_each_possible_cpu(cpu) {
		ct = per_cpu(nft_ct_pcpu_template, cpu);


		if (!ct) >> 			break;
		per_cpu(nft_ct_pcpu_template, cpu) = NULL;

Syzbot hit a UAF in nft_ct_tmpl_put_pcpu() (*), but freed template should be

So I suspect following scenario:

CPU0:			CPU1:
= per_cpu()
			= per_cpu()

per_cpu = NULL
			* UAF *

Hi, Florian!

Yes and no.  The above is fine since pcpu will return different pointers
for cpu 0 and 1.

Dumb question: why per_cpu() will return 2 different pointers for CPU 1 and CPU 0? As I understand for_each_possible_cpu() will iterate over all CPUs which could ever be enabled. So, we can hit situation when 2 concurrent processes call per_cpu() with same cpu value (*).

The race is between two different net namespaces that race when
changing nft_ct_pcpu_template_refcnt.
This happens since

commit f102d66b335a417d4848da9441f585695a838934
netfilter: nf_tables: use dedicated mutex to guard transactions

Before this, all transactions were serialized by a global mutex,
now we only serialize transactions in the same netns.

Its probably best to add
DEFINE_MUTEX(nft_ct_pcpu_mutex) and then acquire that when we need to
inc/dec the nft_ct_pcpu_template_refcnt so we can't have two distinct
cpus hitting a zero refcount.

Would you send a patch for this?

Anyway, I think, moving locking a bit higher is good here, let's test it. I will prepare a patch, if it will pass syzbot testing, thanks!

#syz test
git:// master

With regards,
Pavel Skripkin

>From 616e99fd3ac738b2b5e43c5bc57f6f8cc7a49da0 Mon Sep 17 00:00:00 2001
From: Pavel Skripkin <paskripkin@xxxxxxxxx>
Date: Tue, 10 Aug 2021 00:13:38 +0300
Subject: [PATCH] netfiler: protect nft_ct_pcpu_template_refcnt with mutex

/* .... */

Signed-off-by: Pavel Skripkin <paskripkin@xxxxxxxxx>
 net/netfilter/nft_ct.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nft_ct.c b/net/netfilter/nft_ct.c
index 337e22d8b40b..99b1de14ff7e 100644
--- a/net/netfilter/nft_ct.c
+++ b/net/netfilter/nft_ct.c
@@ -41,6 +41,7 @@ struct nft_ct_helper_obj  {
 static DEFINE_PER_CPU(struct nf_conn *, nft_ct_pcpu_template);
 static unsigned int nft_ct_pcpu_template_refcnt __read_mostly;
+static DEFINE_MUTEX(nft_ct_pcpu_mutex);
 static u64 nft_ct_get_eval_counter(const struct nf_conn_counter *c,
@@ -525,8 +526,10 @@ static void __nft_ct_set_destroy(const struct nft_ctx *ctx, struct nft_ct *priv)
 	case NFT_CT_ZONE:
+		mutex_lock(&nft_ct_pcpu_mutex);
 		if (--nft_ct_pcpu_template_refcnt == 0)
+		mutex_unlock(&nft_ct_pcpu_mutex);
@@ -564,9 +567,13 @@ static int nft_ct_set_init(const struct nft_ctx *ctx,
 	case NFT_CT_ZONE:
-		if (!nft_ct_tmpl_alloc_pcpu())
+		mutex_lock(&nft_ct_pcpu_mutex);
+		if (!nft_ct_tmpl_alloc_pcpu()) {
+			mutex_unlock(&nft_ct_pcpu_mutex);
 			return -ENOMEM;
+		}
+		mutex_unlock(&nft_ct_pcpu_mutex);
 		len = sizeof(u16);

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